Devices having fragile envelopes partly filled with mercury



A. J. YOUNG Oct. 9, 1956 DEVICES HAVING FRAGILE ENVELOPES PARTLY FILLED WITH MERCURY Filed May 26, 1953 DEVICES HAVING FRAGILE ENVELGPES PARTLY FILLED WITH MERCURY Arthur James Young, Danbury, England, assignor to English Electric Valve Company Limited, London, England, a company of Great Britain Application May 26, 1958, Serial No. 357,660

Claims priority, application Great Britain July 22., 1952 2 Claims. (Cl. 313-16'4) This invention relates to devices having glass or like filtrates Patent envelopes partly filled with mercury. Although the invention is primarily intended for the igniter bulbs of mercury pool switches it is not limited to this particular application and may be used in all cases where there is an envelope made of glass or other fragile material partly filled with mercury so as to leave a considerable space not so filled.

A defect which has been experienced with the igniter bulbs of mercury pool switches is that however carefully they may be packed, a considerable number of breakages occur in transport. This has been found to be due to the mercury inside the igniter envelope. If such a device is suddenly inverted or subjected to shock the mer cury moves about more or less violently inside the igniter envelope, and due to its weight may, and often does, hammer the envelope hard enough to break it. This defect is also present, though not quite to so strong a degree, in known sealed contact devices employing liquid mercury wherein the mercury, instead of being free within the envelope and held by the walls thereof, is retained in a given location within the envelope by a mercury wettable sponge occupying only a small volume in the envelope and well spaced from the walls thereof. If such a device is submitted to violent shaking, the mercury, which is only held in the sponge by surface tension, will shake free and may break the envelope by being projected, unbaflled, against part of the interior surface of the wall thereof.

According to this invention a device having a glass or another envelope made of fragile material and partly filled with mercury is provided inside said envelope with gauze made of a material which does not dissolve in mercury and does not otherwise interfere with the intended use of the device, said gauze being arranged to act as a baffle against violent movement of the mercury inside the envelope.

The invention is illustrated in the accompanying drawings in which Fig. 1 shows in cross section a well known form of igniter bulb and Fig. 2 schematically shows the same device modified to be in accordance with this invent-ion.

Referring first to Fig. l the bulb therein shown comprises a glass envelope 1 having a neck 6 into which projects an electrode 2. The envelope contains a pool of mercury 3 which about half fills the bulb and which is connected through lead 7 to the exterior of the bulb. It is found that, however carefully such devices are packed for transport, a considerable number of breakages take place, the break-ages usually occurring in the or 2,765,396 Egg idatented Oct. 9, 1956 wall 4 of the neck. This is due to the inertia elfect of the mercury which is able in suitable circumstances to rush into the neck, and hammer against and break the walls thereof.

In accordance with this invention and as shown in Fig. 2 there is placed inside the envelope a coil or roll 5 of metallic gauze which projects into the neck 6 in insulated relation to electrode 2. With this provision, if the igniter bulb is suddenly inverted, the mercury is forced through the holes in the gauze which baffles its movements so that it falls much more slowly than it would do if the gauze were not present. In this way the occurrence of massive hammer blows by the mercury is prevented and the device is much better able to withstand normal rough handling in transport. Indeed it'has been found that a device as shown in Fig. 2, properly packed, will usually withstand a five foot drop.

The gauze 5" may be of any suitable material which does not dissolve in mercury e. g. iron or nickel. The gauze may with advantage be wound into a coil or roll with most turns in that part which is in the lower half of the bulb, i. e. that half away from the neck, and the gauze of the roll may be bent and deformed in haphazard fashion in this half of the bulb so as more or less to spread over all the volume of said half as conventionally indicated in Fig. 2 in insulated relation to electrode 2.

The invention is of wide application to all devices with fragile envelopes containing substantial quantities of free mercury and the nature of the baffling material employed should be suitably chosen in each case to suit the purpose of the device to which the invention is to be applied i. e. to be such that it will not interfere with the intended use of the device. In general this requirement is satisfied if the gauze maintained in insulated spaced relation to electrode 2 and is such as not to dissolve in mercury.

I claim:

1. A discharge tube comprising an envelope formed from fragile material and including a substantially cylindrical re-entrant portion in one end thereof, said cylindrical re-entrant portion being annularly spaced from the interior end wall of said envelope, a volume of free mercury within said envelope and having part of the internal surface of said envelope as a boundary thereof, said volume of free mercury being substantially less than the internal volume of said envelope, an electrode extending axially through the re-entrant portion of said en velope, and a substantially cylindrical baflie formed of gauze insoluble in mercury surrounding and supported at one end by the re-entrant portion of said envelope and spaced from both the interior end wall thereof and from said axially extending electrode, said bafile permitting free movement of said volume of free mercury While prevent ing violent movement thereof.

2. A discharge tube as set forth in claim 1 in which the end of said baffle opposite the end which is supported by the re-entrant portion of said envelope is bent and deformed in haphazard fashion and spread over substantially the entire volume of the interior of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 

